The present invention relates to a micro-device with an element which deforms under the effect of a thermal actuator. This micro-device may constitute a microswitch which is particularly well-suited to switching of radio frequency signals.
Microswitches are micro-devices which are increasingly used in modern electronic devices one of the major characteristics of which is their increasingly small size. This is the case, notably, with mobile telephones. The design of a microswitch for this type of equipment is confronted with the delicate problem of the on-board available power to activate the microswitches. Current microswitches must be able to be controlled using low voltages (3V for example) and over very short times.
The document xe2x80x9cMicromechanical relay with electrostatic actuation and metallic contactsxe2x80x9d by M. -A. GRETILLAT et al., Transducers ""99, Jun. 7-10, 1999, Sendai, Japan, divulges an electrostatically-controlled microswitch requiring a control of around 20 V.
The document xe2x80x9cBulk micromachined relay with lateral contactxe2x80x9d by Zhihong LI et al., published in J. Micromech. Microeng. 10 (2000), pages 329-333, divulges an electrostatically controlled relay using large facing areas. This causes a pneumatic dampening. The system is dampened and the switching times increase. Moreover, technical production of the active line""s contact is very difficult and the large number of electrodes involved tends to cause disturbances in the control on the radio frequency signal conveyed by the active line.
Document FR-A-2 772 512 divulges a micro-system, usable notably to produce microswitches or micro-valves, constituted on a substrate and used to obtain triggering between a first operational state and a second operational state by means of a thermal actuator with bimetallic effect. The actuator comprises a deformable element attached, by opposite ends, to the substrate so as to present naturally a deflection without constraint compared to a surface of the substrate opposite it; this natural deflection determines the first operational state, and the second operational state is caused by the thermal actuator which, under the effect of a temperature variation, causes a deformation of the deformable element tending to reduce its deflection and subjecting it to a compression stress which causes triggering of it by a buckling effect in a direction opposite to its natural deflection. This device requires a relatively major thermal exchange to control it. When the control resistor is heated the member constituting the deformable element dissipates a large proportion of the heat produced (by radiation and conduction). This energy loss must be taken into account in calculating the energy to be applied for control of the bimetallic element. Moreover the structure""s trigger time is relatively long as a consequence of the time required for thermal conduction and also as a consequence of the losses by radiation with the environment which must be compensated during heating.
To remedy the disadvantages mentioned above, a micro-device is proposed comprising conductors located on a first level and conductors located on a second level, where the conductors of the first level are supported by a deformable element which can trigger by means of an actuator with bimetallic effect; the effect of the triggering is to modify the gap between the conductors on the first level and the conductors on the second level, characterised in that the actuator with bimetallic effect consists of resistors in close and localised contact with the deformable element, and in that the resistors are able, when traversed by an electric control current, to expand sufficiently under the effect of the heat produced by the passage of the electric control current to cause, by a bimetallic effect, a triggering of the deformable element before the heat produced in the resistors has been able to propagate in the deformable element.
The deformable element is preferably a member or a membrane.
Electrostatic holders may be included to hold the deformable element in the position it has after it is triggered, when the control current is cancelled. The electrostatic holders may include at least one pair of electrodes facing one another, with one of these electrodes forming a single piece with the deformable element, and the other being located such that, when the deformable element has triggered, the gap between the facing electrodes is minimal.
In one variant embodiment, the electrostatic holders include at least one pair of facing electrodes, with one of these electrodes forming a single piece with the deformable element, and the other being located such that, when the deformable element has triggered, the electrodes are in contact with one another but separated by electrical insulators.
The resistors may include at least one layer deposited in the shape of a wave. This leads to improved efficiency for the actuator.
The resistors are preferably made from a material chosen from aluminium, manganese, zinc, gold, platinum, nickel or inconel 600.
If the micro-deposit is accomplished using micro-technology techniques, the deformable element may originate from a layer deposited on a substrate.
In a first embodiment, the conductors located on the second level include a first line contact and a second line contact, and the effect of triggering the deformable element is to reduce to zero the distance between the conductors on the first level and the conductors on the second level, with the first level conductors thus forming an electrical link between the first contact and the second contact, and the micro-device thus constituting a microswitch. The conductors supported by the deformable element are ideally constituted by a conductive block.
In a second embodiment, the first level conductors and the second level conductors respectively constitute a first electrode and a second electrode of a condenser, and where this condenser has a first capacity value before the triggering of the deformable element and a second capacity value after the triggering of the deformable element.
According to one variant embodiment, an insulating layer of high dielectric constant separates the first electrode and the second electrode of the condenser. This insulating layer, of thickness less than 0.1 xcexcm for example, may be located on one of the two electrodes, or on both of them.